Clean up redundant re-exports

[Bromeon]

Symbols are often accessible from a large number of modules.

Example Vector3 (a manually declared type):

Example Node (a GDNative auto-generated type):

This is confusing as to which symbol is "the right one" and adds redundancy without benefit. It can also make auto-complete in IDEs harder to use and leads to inconsistent use statements.

Goals:

• For each symbol, work out one canonical module (e.g. Vector3 -> core_types or core_types::geom)
• Remove all non-canonical, non-prelude modules, in particular:
  • Ones existing only due to file structure, as in gdnative::api::node::Node
  • Nested ones inside prelude
• Review prelude (possibly out of scope)

This may break use statements in client code and will thus target v0.10.

[jacobsky]

@Bromeon I'll look into working on this change.

[Bromeon]

As mentioned on the Discord, I already had some changes lying around. I rebased the onto master and although unfinished, I pushed a first version, which we can use as a base for discussion.